Method of manufacturing gold thin film by using electroless-plating method

ABSTRACT

Provided is a method of manufacturing a gold (Au) thin film on a dielectric surface by using an electroless-plating method, the method including: manufacturing a reaction mixture by adding an Au chloride compound and an alkaline compound to an alcohol-water mixed solution; and forming an Au thin film by putting a substrate in the reaction mixture and stirring the reaction mixture. Accordingly, compounds that are relatively low toxic may be used as raw materials, and an Au thin film having a surface enhancement Raman scattering (SERS) effect may be conveniently and stably formed on a dielectric surface without having to use expensive additional equipment, such as a vacuum device.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2013-0100496, filed on Aug. 23, 2014, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a gold (Au) thin film on a dielectric surface, and more particularly, to a method of manufacturing an Au thin film on a glass surface, which has increased activity of surface enhanced Raman scattering (SERS) by using an electroless-plating method.

2. Description of the Related Art

Generally, a gold (Au) thin film is manufactured by using one of three methods, i.e., an electroplating or electro-deposition method, a vapor deposition method, and an electroless-plating method.

In the electro-deposition method, elaborate and expensive equipment is required to guarantee deposition at an accurate ratio and a suitable electric potential. Moreover, in the electro-deposition method, an electric contact should be performed on a plating surface. In addition, not only a long time is taken for such an electric contact if an integrated circuit (IC) having a very complex circuit pattern, specially a certain high density, is used. Furthermore, a surface that is plated should be conductive, and should be connected to an external power source.

Also, even the vapor deposition method has a few intrinsic weaknesses. In various application fields, elaborate and high vacuum equipment is required, and a large amount of Au metal is consumed during an evaporation process. In addition, it is difficult to attach evaporated Au only to a selected area in a plated surface. In other words, it is not easy to design a pattern having Au by using the vapor deposition method.

Effects of surface enhanced Raman scattering (SERS) may increase according to changes in a surface or structure of a metal. In detail, according to recent studies, SERS may be detected even in a single molecule. A technology using such SERS is applied to various molecular electronics, for example, to a chemical analyte, an etching agent, a lubricant, a catalyst, and a sensor.

Meanwhile, KR1277357 provides a layer having a barrier function and catalytic power and excelling in formation uniformity and coverage of an ultrathin film, provides a pretreatment technique wherein an ultrafine wiring can be formed and a thin seed layer of uniform film thickness can be formed, and discloses a substrate including a thin seed layer formed with a uniform film thickness via electroless plating by using the pretreatment technique.

SUMMARY OF THE INVENTION

The present invention provides a method of manufacturing a gold (Au) thin film conveniently and stably having an effect of surface enhancement Raman scattering (SERS) activity by using an electroless-plating method, without having to not only use expensive equipment but also perform an additional process.

According to an aspect of the present invention, there is provided a method of manufacturing a gold (Au) thin film by using an electroless-plating method, the method including: manufacturing a reaction mixture by adding an Au chloride compound and an alkaline compound to an alcohol-water mixed solution; and forming an Au thin film by putting a substrate in the reaction mixture and stirring the reaction mixture.

The alcohol-water mixed solution may be a mixed solution containing 70 to 90 wt % of alcohol and 30 to 10 wt % of water.

The alcohol may be C1 to C4 alcohol, and in detail, may be methanol or ethanol.

The Au chloride compound may be selected from the group consisting of potassium Au chloride (KAuCl₄), gold potassium cyanide (KAu(CN)₂), and chloroauric acid (HAuCl₄).

The alkaline compound may be selected from the group consisting of potassium carbonate, sodium hydroxide, potassium hydroxide, butylamine, and sodium hydrogen carbonate.

The substrate may be formed of a dielectric material selected from the group consisting of glass, plastic, and silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

FIG. 1 is field emission scanning electron microscopic (FE-SEM) images of gold (Au) thin films deposited on glass substrates according to reaction times of 20 minutes, 40 minutes, 60 minutes, and 80 minutes, respectively;

FIG. 2 illustrates ultraviolet-visible (UV-vis) absorption spectra of the Au thin films deposited on the glass substrates according to the reaction times of 20 minutes, 40 minutes, 60 minutes, and 80 minutes;

FIG. 3 (a) illustrates X-ray diffraction (XRD) patterns of the Au thin films deposited on the glass substrates according to the reaction times of 40 minutes and 80 minutes, and FIG. 3 (b) illustrates an X-ray photoelectron spectroscopy (XPS) spectrum of the Au thin film deposited on the glass substrate according to the reaction time of 80 minutes;

FIG. 4 (a) illustrates surface enhancement Raman scattering (SERS) spectra of benzenthiol (BT) adsorbed on the Au thin film deposited on the glass substrates, and FIG. 4 (b) illustrates relative Raman peak intensities of BT at 1574 cm⁻¹ adsorbed on the Au thin films deposited on the glass substrates; and

FIG. 5 illustrates SERS spectra of BT at 1574 cm⁻¹ adsorbed on five different batches A through E of an Au film.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a method of manufacturing a gold (Au) thin film on a dielectric surface by using an electroless-plating method, according to one or more embodiments of the present invention, will now be described in detail.

First, a reaction mixture is manufactured by adding an Au chloride compound and an alkaline compound to an alcohol-water mixed solution. Here, alcohol is a reducing agent that supplies electrons.

After putting the substrate into the reaction mixture, the reaction mixture is stirred to form an Au thin film.

The alcohol-water mixed solution may contain 70 to 90 wt % of alcohol and 30 to 10 wt % of water.

Also, the alcohol may be C1 to C4 alcohol, and in detail, may be methanol or ethanol. The Au chloride compound may be selected from the group consisting of potassium Au chloride (KAuCl₄), gold potassium cyanide (KAu(CN)₂), and chloroauric acid (HAuCl₄), and the alkaline compound may be selected from the group consisting of potassium carbonate, sodium hydroxide, potassium hydroxide, butylamine, and sodium hydrogen carbonate.

The substrate may be formed of a dielectric material selected from the group consisting of glass, plastic, and silicon.

As such, if the Au chloride compound and the alkaline compound are put into the water-alcohol mixed solution that is methanol or ethanol and a temperature is maintained to 50° C. to 70° C., Au nanoparticles adhere to any one of various dielectric surfaces, such as glass, silicon, and plastic surfaces, and sizes of the Au nanoparticles or a thickness of the Au thin film may be adjusted by varying the concentrations of reactants or by adjusting a reaction time.

A state of the Au thin film adhered on the dielectric surface as such was analyzed by using an ultraviolet-visible (UV-vis) spectrum analyzer, a field emission scanning electron microscope, an X-ray diffraction (XRD) analyzer, and an X-ray photoelectron spectroscopy (XPS) analyzer, and as results, it was determined that the Au thin film adhered on the dielectric surface was formed as Au particles having nanometer sizes gather together, and sizes of the Au particles and a thickness of the Au thin film depend on a reaction time.

Also, the Au thin film manufactured by using the electroless-plating method, according to an embodiment of the present invention, showed uniform surface enhancement Raman scattering (SERS) activity on a surface up to hundreds of thousands of square micrometers, and an enhancement factor (EF) calculated by using benzenthiol (BT) as a prototype adsorbent reached up to 7.6×10⁴.

Example 1 Manufacturing Au Thin Film

A cover glass having a diameter of 18 mm (manufactured by Marienfeld) was soaked in an alkaline cleaning solution (pH=9.2; and 0.5% Hellmanex II, Hellma) for 3 hours and then was sonicated in distilled water for 10 minutes. Next, the cover glass was washed with ethanol and then was finally dried in an oven at a temperature of 60° C. for 30 minutes. The washed cover glass was dipped in a reaction mixture and then the reaction mixture was vigorously stirred at a temperature of 50° C. Here, the reaction time was varied from 20 minutes to 80 minutes. The reaction mixture was obtained by mixing an aqueous solution containing 0.5 mL of 0.1 M HAuCl₄ and 1 mL of 1 M K₂CO₃ with 8.5 mL of methanol solution, and at this time, the pH of the reaction mixture was adjusted to 11 to 12. An Au-coated glass obtained as such was washed with ethanol and then dried in air.

For self-assembly of BT on the Au-coated glass, the Au-coated glass was immersed in a 10 mM methanol solution of BT for 30 minutes, washed with deionized water several times, and then dried in air.

Example 2 Analyzing Properties of Au Thin Film

1. Methods of Analyzing Properties

UV-vis spectra were obtained by using a spectrum analyzer (Avantes 3648), and field emission scanning electron microscopic (FE-SEM) images were obtained by using a field emission scanning electron microscope (JSM-6700F) that operated at 5.0 kV.

XRD was analyzed by using an X-ray diffractometer (Rigaku Model MiniFlex powder diffractometer) using Cu K_(α) radiation. Also, XPS measurements were performed by using an AXISH model using Mg Kα X-ray as a light source.

SERS was analyzed by using a spectroscope (Renishaw Raman System Model 2000) equipped with an integral microscope (Olympus BH2-UMA). The 632.8 nm line from a 17 mW He/Ne laser (Spectra Physics Model 127) was used as an excitation source. A Raman band of a silicon wafer at 520 cm⁻¹ was used to calibrate the spectroscope. Accuracy of a measured spectrum value was at least 1 cm⁻¹.

Atomic force microscopic (AFM) images were obtained by using an atomic force microscope (Instruments Nanoscope IIIa system), and at this time a nominal spring constant was from 20 N/m to 100 N/m, and an 125 μm long etched cantilever was used. Topographic images were recorded in a tapping mode under an operating frequency within 300 kHz at a scanning speed of 2 Hz.

2. Experiment Results

FIG. 1 is FE-SEM images of Au thin films deposited on glass substrates according to reaction times, for example, 20 minutes, 40 minutes, 60 minutes, and 80 minutes, respectively. An average particle size of the Au thin film according to the reaction time of 20 minutes is 22.8±3.8 nm, an average particle size of the Au thin film according to the reaction time of 40 minutes is 176±18 nm, and as the reaction time increases, Au nanoparticles coalesce into large grains to form a network structure and cover an entire surface of the glass substrate. Meanwhile, it is difficult to determine grain sizes of the Au thin films according to the reaction times of 60 minutes and 80 minutes.

FIG. 2 illustrates UV-vis absorption spectra of the Au thin films deposited on the glass substrates according to the reaction times, for example, 20 minutes, 40 minutes, 60 minutes, and 80 minutes. The Au thin film according to the reaction time of 20 minutes shows maximum absorption at 534 nm, and the Au thin films according to the reaction times of 40 minutes, 60 minutes and 80 minutes show maximum absorption at 563 nm, 584 nm, and 639 nm, respectively.

FIG. 3 (a) illustrates XRD patterns of the Au thin films deposited on the glass substrates, wherein XRD peaks positioned at 38.2°, 44.4°, 64.6°, and 77.6° respectively correspond to (111), (200), (220), and (311) lattice planes of face centered cubic gold particles.

FIG. 3 (b) illustrates an XPS spectrum of the Au thin film deposited on the glass substrate according to the reaction time of 80 minutes, wherein XRD peaks at 83.7 eV and 87.4 eV respectively correspond to 4f _(7/2) and 4f _(5/2) peaks of zero-valent Au.

FIG. 4 (a) illustrates SERS spectra of BT adsorbed on the Au thin films deposited on the glass substrates, wherein bands at 998 cm⁻¹, 1021 cm⁻¹, 1072 cm⁻¹, and 1573 cm⁻¹ respectively correspond to an in-plane ring-breathing mode, an in-plane C-H bending mode, an in-plane ring-breathing mode coupled with a C-S stretching mode, and a C-C stretching mode,

FIG. 4 (b) illustrates relative Raman peak intensities of BT at 1574 cm⁻¹ adsorbed on the Au thin films deposited on the glass substrates, wherein the Au thin film according to the reaction time of 60 minutes shows the most intense SERS peak of BT, whereas the Au thin films according to the reaction times of 20 minutes and 80 minutes show very weak SERS peaks. Such results highlight the importance of gaps and crevices in a metal nanostructure in SERS measurements.

Also, an SERS EF may be calculated according to Equation 1 below.

EF=(I _(SERS) /I _(NR))(N _(NR) /N _(SERS))  (1)

Here, I_(SERS) and I_(NR) respectively denote SERS intensity of BT on an Au thin film and normal Raman (NR) scattering intensity of BT in a bulk. N_(NR) and N_(SERS) denote numbers obtained by illuminating BT molecules by using a laser beam to respectively obtain SERS and NR spectra.

I_(SERS) and I_(NR) were measured at 1574 cm⁻¹, and N_(NR) and N_(SERS) were calculated based on estimated concentration of surface BT species, density of bulk BT, and sampling areas. It was assumed that equilibrated surface concentration of BT is the same as that of Au and silver (Ag), i.e., up to 7.1×10⁻¹⁰. N_(SERS) was calculated to be 1.0×10⁻¹⁷ mol considering a sampling area having a diameter of 1 μm and a surface roughness factor (up to 1.47) obtained from AFM measurement of the thin film according to the reaction time of 60 minutes. When an NR spectrum of pure BT was measured, a sampling volume was a product of a laser spot and a penetration depth was about 15 μm. N_(NR) was 1.1×10¹³ mol when density of BT was 1.07 g/cm³.

Since an intensity ratio of I_(SERS)/I_(NR) was measured to be up to 6.9 at 632.8 nm excitation, EF may be as large as 7.6×10⁴. The calculated EF was in fact comparable to 10⁶ for pyridine adsorbed onto an electrochemically roughened Au surface. In addition, the SERS spectra on the Au films were proved to be highly reproducible.

FIG. 5 illustrates SERS spectra of BT at 1574 cm⁻¹ adsorbed on five different batches A through E of the Au film according to the reaction time of 60 minutes. Peak intensities at 1574 cm⁻¹ were normalized with respect to silicon wafers, and batch-to-batch relative deviation was 15% whereas spot-to-spot relative deviation was 12%.

Accordingly, an Au thin film manufactured by using an electroless-plating method, according to an embodiment of the present invention, shows uniform SERS activity on an area up to hundreds of thousands of square micrometers, and an EF calculated by using BT as a prototype adsorbent reached up to 7.6×10⁴.

By using the method described above, an Au thin film having an effect of SERS activity may be conveniently and stably formed on a dielectric surface.

According to the method of manufacturing an Au thin film on a dielectric surface by using an electroless-plating method of the present invention, compounds that are relatively low toxic may be used as raw materials, and an Au thin film having an SERS effect may be conveniently and stably formed on a dielectric surface without having to use expensive additional equipment, such as a vacuum device. In detail, since a thickness of an Au thin film formed on a dielectric substrate, such as glass, may be adjusted, the Au thin film may be applied to various fields, such as semiconductor fields, energy fields, catalyst fields, medical fields, and diagnosis fields.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. 

1. A method of manufacturing a gold (Au) thin film by using an electroless-plating method, the method comprising: manufacturing a reaction mixture by adding an Au chloride compound and an alkaline compound to an alcohol-water mixed solution; and forming an Au thin film by putting a substrate in the reaction mixture and stirring the reaction mixture in the temperature range of 50˜70° C.
 2. The method of claim 1, wherein the alcohol-water mixed solution is a mixed solution containing 70 to 90 wt % of alcohol and 30 to 10 wt % of water.
 3. The method of claim 1, wherein the alcohol is C1 to C4 alcohol.
 4. The method of claim 1, wherein the Au chloride compound is selected from the group consisting of potassium Au chloride (KAuCl4), gold potassium cyanide (KAu(CN)2), and chloroauric acid (HAuCl4).
 5. The method of claim 1, wherein the alkaline compound is selected from the group consisting of potassium carbonate, sodium hydroxide, potassium hydroxide, butylamine, and sodium hydrogen carbonate.
 6. The method of claim 1, wherein the substrate is formed of a dielectric material selected from the group consisting of glass, plastic, and silicon.
 7. The method of claim 2, wherein the alcohol is C1 to C4 alcohol. 